Technical Field
Our invention relates to digital communication systems and more particularly to arrangements for improving the quality of signals carried over a noise-prone digital channel.
Digital communication systems are adapted to carry any signal that can be coded in digital form. Data signals already in digital form must be converted to the format of the digital facility over which they are to be transmitted. Analog signals such as speech require sampling and encoding prior to application to the digital facility. In order to take advantage of the inherent wide bandwidth of digital channels, a plurality of coded signals are multiplexed. The multiplexed coded signals are transmitted over a common digital channel. Demultiplexing and decoding of the separated signals at receiving points of the digital facility completes the communication process. While digital encoding improves the signal to noise ratio of the signal transmission, noise introduced on the digital transmission channel may seriously affect the quality of transmission. Impulse type noise, in particular, can seriously modify the bit pattern of one or more digital codes so that the information decoded at the digital facility receiving point is incorrect or of reduced intelligibility.
Many schemes have been devised to overcome the effect of channel noise. In one such arrangement, the coded data signals are divided into blocks, and an algebraically related special code word is added to each block to permit correction of errors. Where such complex algebraic error coding is utilized, the protection code may comprise a set of binary parity checks over preassigned subsets of data bits. The parity checks may be utilized to both detect and correct errors in data bits of the block. Such error coding schemes are applicable to any type of data and to any data source. A parity coding error correcting scheme for audio digital recording, for example, is disclosed in U.S. Pat. No. 4,145,683 issued Mar. 20, 1979 to Marshall R. Brookhart. In the system described in the Brookhart patent, a digitized audio signal is partitioned into frames. Parity words and error check words are generated for each frame. The successive frames are recorded on a record medium. Individual errors are detected and corrected during playback of the frames responsive to the parity and error check words.
While correction of individual errors may be important in data transmission or high quality television or audio transmission, it is not necessary in speech signal transmission where only intelligibility and perceptual acceptability are needed. An error reducing device for improvement of rather than correction of a digitally transmitted signal is disclosed in U.S. Pat. No. 3,386,079 issued May 28, 1968 to M. J. Wiggins. The Wiggins arrangement is operative to transfer previously received information if, during a sampling period, more than one intelligence bearing pulse is detected. In this manner, a degree of noise immunity is obtained although there is no correction of errors due to noise.
U.S. Pat. No. 4,054,863 issued Oct. 18, 1977 to D. J. Goodman and R. Steele and assigned to the same assignee discloses an error detection and correction system in which a sequence of received signals is divided into blocks and each received block is partitioned into subblocks in a digital signal receiver. For each received block, a signal representative of the deviations of the coded signals of the block is generated along with a signal representative of the deviations among the signals of each subblock of the block. When the received subblock deviation signal exceeds the received block deviation signal, the subblock signals are altered to reduce the subblock deviations.
The Goodman et al arrangement is operative to detect abnormal deviations and to provide subgroup deviation reduction. It is apparent, however, that the Goodman arrangement does not correct signal errors but modifies subgroups if errors are thought to be present due to abnormal variations in received signal deviations. Thus, it is likely to modify error free subgroup signals whose deviations appear abnormal. Such "false alarm" error detection and incorrect modification of error free signals severely affects the quality of the signal replica produced by the digital signal receiver. It is an object of the invention to provide an improved error reduction arrangement for the transmission of signals over a digital communication facility.